FIG. 27 is a block diagram illustrating a constitution of a conventional electronic still camera.
Referring to reference numerals in the drawing, 111 denotes an optical lens comprising a Rom 111a in which a shading correction coefficient is stored, 112 denotes an IR cut filter for transmitting only a light having a wavelength equal to lower than a near infrared ray, 113 denotes a CCD image sensor as an imaging element for obtaining an imaging signal through a photoelectric conversion, 114 denotes an analog signal processing circuit for executing a correlate double sampling (CDS), a signal amplification and the like, 115 denotes an A/D converter for converting the imaging signal into a digital signal, 116 denotes a digital signal processing circuit for obtaining a video signal by synthesizing (synchronizing) the imaging signal from the image sensor 113 for each of different chrominance signals, 117 denotes a drive circuit for generating a drive pulse of the image sensor 113, 118 denotes a reference voltage operation circuit for converting the shading correction coefficient read from the ROM 111a into a reference voltage of the A/D converter 115, 119 denotes a D/A converter for converting a digital signal outputted from the reference voltage operation circuit 118 into an analog amount and supplying the analog amount to the A/D converter 115 as the reference voltage, and 120 denotes a memory card for storing the video signal obtained in the digital signal processing circuit 116.
The correction coefficient is read from the Rom 111a according to an address supplied from the drive circuit 117, the reference voltage to be supplied to the A/D converter 115 is calculated in the reference voltage operation circuit 118, and the reference voltage is supplied to the A/D converter 115 in the form of the analog amount so as to execute the shading correction (see No. 2000-41179 of the Publication of the Unexamined Japanese Patent Applications/p. 2-3 and FIG. 6).
As shown in FIG. 28A, if there is a sufficient distance between the optical lens 111 and the sensor 113, incident angles of lights R0, R1 and R2 transmitting through the optical lens 111 relative to the IR cut filter 112 are less different to one another. As shown in FIG. 29, a transmission property of the IR cut filter 112 is different depending on the incident angle, however, the difference, which is hardly detectable, is within an allowable range.
However, in recent years, the optical lens 111 and the sensor 113 are arranged to be increasingly closer to each other as shown in FIG. 28B as the downsizing is continuously promoted. Because of the trend, the incident angle of the light relative to the IR cut filter 112 becomes smaller as a distance relative to an optical axis increases. FIG. 30 shows the transmission property of the IR cut filter per wavelength, wherein a band of the transmittable wavelength shifts to a shorter-wavelength side as the distance relative to the optical axis increases. The foregoing phenomenon results in the generation of the imaging signal reddish at a screen center and bluish in a peripheral part of the screen as shown in FIG. 31.
The conventional shading correction is executed based on information such as a position, an aperture, a type and the like of the lens. When the information is fixed to constant values, the shading correction also shows a constant result. However, the foregoing red component is biased depending on the color temperature of the light entering the camera, which is, however, an independent factor apart from the position, aperture, type and the like of the lens. Therefore, the color temperature cannot be appropriately corrected.
In the case of the imaging signal which is not shading-corrected, if the color temperature is corrected based on the imaging signal in the vicinity of the optical axis, a balance is lost in the blue direction in accordance with the increase of the distance relative to the optical axis. On the contrary, the balance is lost in the red direction in the vicinity of the optical axis when the color temperature is corrected based on the imaging signal distant from the optical axis. In either of the foregoing cases, a quality of the imaging signal is deteriorated.
Further, in the case of AF (auto focus) in which a focal point is estimated based on the imaging signal obtained from the image sensor so as to move the lens, when the focal point is estimated based on the imaging signal in which the color temperatures are not corrected, a sampling intensity is biased relative to only the red component, which makes it difficult to estimate a preferable value. As a result, a desired AF operation cannot be realized.